High temperature ultraviolet radiation detector

ABSTRACT

An ultraviolet sensitive, gaseous discharge detector of the Geiger-Muller type has a low spurious count rate and a prolonged lifetime at elevated temperatures. The gas filling of the detector comprises a gas mixture of neon and argon with a small amount of hydrogen added. The gas mixture of neon and argon substantially determines the breakdown voltage and ionization efficiency characteristics of the gas filling. The essential function of the hydrogen is to rapidly de-excite argon metastable atoms which would otherwise cause excessive spurious discharge counts.

United States Patent Johnson Oct. 23, 1973 HIGH TEMPERATURE ULTRAVIOLET RADIATION DETECTOR Robert G. Johnson, Minnetonka, Minn.

Inventor:

Assignee: Honeywell lnc., Minneapolis, Minn.

Filed: June 29, 1972 Appl. No.: 267,269

1m. (:1. 1101, 39/08 Field of Search 313/100, 101, 214, 313/226, 93; 250 836 References Cited UNITED STATES PATENTS 9/1967 Engh et al 313/100 Primary Examiner-Roy Lake Assistant ExaminerDarwin R. Hostetter AttorneyLamont B. Koontz et al.

[57] ABSTRACT An ultraviolet sensitive, gaseous discharge detector of the Geiger-Muller type has a low spurious count rate and a prolonged lifetime at elevated temperatures. The gas filling of the detector comprises a gas mixture of neon and argon with a small amount of hydrogen added. The gas mixture of neon and argon substantially determines the breakdown voltage and ionization efficiency characteristics of the gas filling. The essential function of the hydrogen is to rapidly deexcite argon metastable atoms which would otherwise cause excessive spurious discharge counts.

4 Claims, 5 Drawing Figures Ne IOO% HIGH TEMPERATURE ULTRAVIOLET RADIATION DETECTOR BACKGROUND OF THE INVENTION This invention is related to an ultraviolet radiation sensitive, gaseous discharge detector of the Geiger- Muller type. In particular, it is directed to an ultraviolet radiation detector havinga low spurious count rate and a prolonged lifetime at elevated temperatures.

A Geiger-Muller detector is a radiation detector having an anode and a cathode disposed in an ionizable gas. When subjected to radiation to which it is sensitive, the detector causes an electron to be present within the electric field established by the anode and the cathode. The electron may originate by photo ionization of the gas, but more commonly originates by photo emission from the cathode surface proximate the anode; that is, from thesensitive area of the cathode. The electron accelerates toward the anode, ionizing the gas, and causing a discharge current to flow. The current is subsequently quenched by means of a quenching mechanism.

R. O. Engh et al., in US. Pat. No. 3,344,302, describes a highly advantageous ultraviolet radiation detector capable of operating at low voltages and having high current carrying capacity while exhibiting minimal spurious count rates. The detector comprises a radiation permeable envelope, a pair of closely spaced electrodes, and a gas filling having a high ionization efficiency in the vicinity of the electrodes and a low ionization efficiency in other regions within the envelope. lonization is thereby promoted in the vicinity of the electrodes and inhibited in other regions within the envelope.

The preferred gas filling described by Engh is a Penning gas mixture of about .85 per cent neon and 15 per cent hydrogen at a total pressure of 100 torr. Other Penning gas mixtures suchas helium-hydrogen and neon-argon-hydrogen are also described as having characteristics similar to the neon-hydrogen mixture. Hydrogen is a highly advantageous component of these Penning gas mixtures because of its fast ion mobility, its ability to quench or de-excite rapidly the noble gas metastable atoms, and its low sputtering rate characteristics. The function of hydrogen in the Penning mixture described by Engh is to de-excite metastable atoms of the major constituent of the mixture.

In certain applications it is highly advantageous to have an ultraviolet detector capable of operation at high temperatures. For example, in flame sensing apparatus for oil burners, a detector capable of reliable high temperature operation allows the detector to be positioned close to the flame, thereby simplifying the mounting apparatus for the detector and enhancing sensitivity to small signals.

It has been found that the detector described by Engh is capable of reliable operation up to temperatures of 750F. At higher temperatures, severe changes occur in the operating characteristics of the detector due to a loss of hydrogen by permeation through the wall of the detector envelope. In particular, the ionization effi ciency characteristics of the gas filling are greatly influenced by the amount of hydrogen present. The average ionization efficiency at all points within the envelope increases rapidly as hydrogen is lost, thereby increasing the probability of spurious count generation caused by electrons emitted from the interior surfaces of the envelope. Only a 30 per cent loss of hydrogen, which may occur within hours or hundreds of hours of high temperature operation, causes effective failure of the prior art detectors.

SUMMARY OF THE INVENTION The ultraviolet radiation detector of the present invention has prolonged lifetime at elevated temperatures as well as a low spurious count rate and low operating voltage. A pair of closely spaced electrodes are positioned in an envelope. Contained in the envelope is an ionizable gas filling having a low breakdown voltage, a high average ionization efficiency in a region proximate the spaced electrodes, and a low average ionization efficiency in other regions within the envelope. The ionizable gas filling includes a gas mixture of neon and one or more of the following: argon, krypton, and xenon. The gas mixture substantially determines the breakdown voltage and ionization efficiency characteristics of the ionizable gas filling. Also included in the ionizable gas filling is a small amount of hydrogen to ensure the adequate de-excitation of argon, krypton, or xenon metastable atoms.

BRIEF DESCRIPTION OF THE DRAWINGS FIG; 1 is a side view partially in section of a detector of the present invention.

FIG. 2 is a graph showing the preferred initial gas fillings of the present invention.

FIG. 3 is a graph showing 17, the ionization efficiency, as a function of E, the electric field, and p,,, the total gas pressure, for a per cent hydrogen gas filling, and showing changes in n with a 40 per cent hydrogen loss.

FIG. 4 is a graph showing changes in 1; as a function of E and p when hydrogen is lost from a hydrogenneon gas filling.

FIG. 5 is a graph showing changes in 1 as a function of E and p as hydrogen is lost from one preferred gas filling of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a preferred embodiment of the radiation detector of the present invention. Envelope 10, formed by quartz or by an ultraviolet transparent silica glass suchas Corning type number 9,741, defines an enclosed volume. Other types of envelopes may also be used such as metal envelopes having transparent windows. Anode 11 and cathode 12 are positioned within the enclosed volume in a relatively closely spaced relationship with respect to one another. A relatively large spaced relationship exists between the two electrodes and the interior surfaces of envelope 10. Molybdenum, nickel, and tungsten are suitable electrode materials for the ultraviolet detector. An ionizable gas filling comprising a mixture of neon arid argon, neon and krypton, neon and xenon or combinations thereof with a small amount of hydrogen added is contained within the enclosed volume. Argon, krypton, and xenon each has a suitable ionization energy level relative to the neon metastable level and therefore may be used in the gas filling of the present invention. Argon, however, will be specifically discussed since argons substantially lower cost and lower sputtering rate make the neon and argon gas mixture the preferred mixture.

The majority of spurious discharges in Geiger-Muller detectors are caused by electrons emitted from interior surfaces such as the envelope walls. To minimize the spurious counts, it is desirable to confine the electric discharge and ionization in the gas filling to the region between the electrodes, and to inhibit discharge and ionization in other regions within the detector envelope. This object is achieved by selection of the proper type of gas filling and the proper spacing of the anode and cathode within the envelope.

The gas filling of the present invention includes either a Penning gas mixture of neon and argon with the majority gas neon, or a similar gas mixture with the majority gas argon. In either case, the gas filling utilizes the Penning ionization mechanism. Metastables of neon ionize argon and hydrogen in the gas filling. The gas filling contains a much larger amount of argon than hydrogen; therefore the breakdown voltage, the oathode sensitive area, and the ionization efficiency at all points within the tube are substantially determined by the neon and argon. Consequently, the loss of hydrogen by permeation changes the breakdown voltage, total gas density, and cathode sensitive area very little, and the detector tube characteristics remain relatively stable. Most important of all, the ionization efficiency of points outside the sensitive area of the cathode is not increased significantly with hydrogen loss; thus the spontaneous emission of electrons from the interior surfaces do not produce many avalanche products that lead to spurious counts.

The important role of hydrogen is to de-excite the argon metastable atoms. In the absence of hydrogen, the argon metastables persist for a long time and cause the discharge to be very difficult to quench. Only a relatively small amount of hydrogen is required to deexcite the argon metastables. Calculations indicate that the average de-excitation time for an argon metastable in torr hydrogen is in the neighborhood of 10 nanoseconds. In 100 of these time intervals the probability of any one argon metastable escaping de-excitation is about 1 in 10. Therefore, in l microsecond all metastables should be de-excited. Since the normal required dead time of the detectors of the present invention is at least one millisecond, argon metastables are not a problem if the partial pressure of hydrogen is above 0.01 torr.

To assure a prolonged lifetime at high temperatures, the initial amount of hydrogen should be sufficient to allow most of the hydrogen to be lost without causing a loss in the ability to adequately de-excite argon metastables. FIG. 2 graphically shows the preferred initial gas fillings of the present invention. The initial amount of hydrogen ranges from about 4 mole per cent to about mole per cent; argon ranges from about 8 mole per cent to about 60 mole per cent; and neon comprises the balance of the initial filling. To ensure that neon and argon substantially determine the breakdown voltage and ionization efficiency characteristics of the filling, the percentage of argon should be at least twice the percentage of hydrogen.

FIG. 2 also shows the most preferred initial gas fillings comprising between about 6 mole per cent and about 12 mole per cent hydrogen, between about mole per cent and about 50 mole per cent argon, and balance neon.

To account for the improvement in lifetime at high temperatures, the effect of hydrogen loss on ionization efficiency characteristics in the detector must be considered. FIG. 3 shows the ionization efficiency '17 as a function of E/P where E is the field strength in volts per centimeter and p, is the gas pressure in torr in a detector tube containing pure hydrogen. Point A on the curve represents the field strength at the edge of the cathode sensitive area in the tube. Point B represents the field strength at a peripheral region in the tube distant from the sensitive area. It can be seen that the ionization efficiency at the peripheral region represented by point B is much less than the ionization efficiency at the edge of the cathode sensitive area represented by point A. A 40 per cent loss of hydrogen by wall permeation causes the ionization efficiency at the edge of the cathode sensitive area to increase to point A. Similarly, the ionization efficiency at the peripheral region point rises to point B. Since the number of photons and other electron-ion avalanche products increases as exp(1 V), where V is the potential drop in the region considered, the increase of n by a factor of two at the peripheral region point makes a very large increase in avalanche products. This in turn causes a large increase in spurious count probability, which is compounded by the increase in cathode sensitivity implied by the rise from point A to point A. This change in ionization efficiency is extremely important since losses of 40 mole per cent of the available hydrogen can occur in a matter of hours or hundreds of hours when the detector tube is operated at high temperatures.

The loss rate of hydrogen by wall permeation is proportional to the hydrogen partial pressure in the prssure ranges of interest. The same fraction of the initial hydrogen quantity is therefore lost in a specified time under similar ambient conditions even in mixtures of gases.

FIG. 4 shows the 20 per cent hydrogenper cent neon ionization efficiency curve. As in FIG. 3, point A represents the field strength at the edge of the cathode sensitive area and point B represents the field strength at a peripheral region in the tube. If the partial pressure of hydrogen is reduced by 60 per cent and any small neon loss is neglected, the total gas density is reduced by 12 per cent. The resulting mixture contains only 9 per cent hydrogen. Points A and B represent the field strength at the edge of the cathode sensitive area and the peripheral region, respectively. The changes in ionization efficiency are comparable to the case of pure hydrogen with 40 per cent loss. It can be seen that only a small advantage in lifetime at high temperature is gained with a 20 per cent hydrogen mixture relative to a per cent hydrogen mixture.

FIG. 5 shows the ionization efficiency curve for a gas mixture of 50 parts neon, 40 parts argon, and 10 parts hydrogen. The shift in ionization efficiency caused by a 60 per cent loss of hydrogen is relatively small, with little change in breakdown voltage, sensitivity, and consequently in spurious counts.

The detector containing the 50-40-10 neon-argonhydrogen gas mixture is not lifetime limited due to hydrogen loss effects on sensitivity, breakdown voltage, or spurious counts caused by peripheral ionization efficiency increases. Rather, the lifetime limit occurs when there is insufficient hydrogen to quench or de-excite argon metastables during the normal dead time interval following each current discharge pulse. This limit occurs at hydrogen concentrations somewhere below 0.01 torr. Assuming that a 30 per cent loss of hydrogen brings a detector containing 80 per cent neon and 20 per cent hydrogen to the failure zone because of increased overall ionization efficiency, the 50-40-10 mixture in the same detector with torr initial hydrogen partial pressure will last at least 19 times as long under similar conditions if unquenched argon metastables provide the lifetime limit.

in one successful embodiment of the present invention, twelve detectors were fabricated containing a gas filling of 50 per cent neon, 40 per cent argon and 10 per cent hydrogen. The anode and cathode were of circular cross section with a diameter of approximately 0.030 inches each and were separated by approximately 0.004 inches. A control group of six detectors containing .the prior art gas filling of 81 per cent neon and 19 per cent hydrogen were simultaneously fabricated. Electrode size and spacing were identical for the two groups of tubes. The average characteristics of the two groups of detectors are as follows:

5 0: l 0. 200v I87 coun s/sec -19 208v 397 counts/sec 43 counts/min 4 counts/min In conclusion, the detector tube of the present invention achieves prolonged lifetime at elevated temperatures by the use of an ionizable gas filling which differs substantially in operation when compared with the prior art gas fillings. Hydrogen de-excites neon metastables in the prior art gas fillings. The loss of hydrogen during exposure to high temperatures causes spurious count failure, because the average ionization efficiency becomes too high as hydrogen is lost, thus permitting spurious electrons to cause counts. Only a 30 per cent loss of hydrogen from the prior art gas filling causes effective failure of the detector. 7

In the gas filling of the present invention, both hydrogen and argon deexcite neon metastables. The amount of argon is much greater than hydrogen, and therefore argon and neon form a gas mixture which substantially determines the breakdown voltage and ionization efficiency characteristics of the ionizable gas filling. The essential function of the small amount of hydrogen is to de-excite argon metastables. At high temperature, the loss of hydrogen by permeation causes spurious count failure only when the hydrogen concentration becomes so low that the argon metastables can no longer be adequately de-excited. During the useful life of the detector, the ionization efficiency remains substantially constant. Effective failure does not occur until hydrogen loss is greater than 99 per cent.

Hydrogen concentration decreases exponentially during high temperature operation. The detector which does not fail until greater than 99 per cent hydrogen is lost has a much greater lifetime than a detector which fails with 30 per cent hydrogen loss. The increase in lifetime is much greater than a factor of three.

In addition to improving the lifetime characteristics of ultraviolet detectors, the gas fillings of the present invention have another advantage. In the design of ultraviolet detector tubes, it is often desirable to use a gas filling having a high total molar density. The use of a high total molar density reduces the ionization efficiency of peripheral regions of the tube, thereby reducing the spurious count rate. The use of a high total molar density is limited, however, because the discharge current density increases as the square of the total molar density. This leads to excessive localized ion bombardment and heating and alteration of the cathode surface.

In the gas fillings of the present invention, a large number of the positive ions carrying the discharge pulse current are argon ions which have much lower mobility under the electric field than the hydrogen ions. Relative to a gas filling in which the hydrogen concentration is greater than that of the argon, a gas filling of this invention having the same total molar density will have a discharge current that has a substantially greater coverage of the cathode, provided that the cathode is not entirely covered by the discharge. This effect is due to the lower mobility of the ion in the present gas fillings, which provides the desired positive ion space charge at lower current densities. Thus the gas fillings of this invention permit the use of higher total molar densities without excessive local bombardment of the cathode.

It is to be understood that this invention has been disclosed with reference to a series of preferred embodiments and it is possible to make changes in form and detail without departing from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 1. An ultraviolet radiation detector characterized by its low spurious count rate and prolonged lifetime at elevated temperatures, the ultraviolet radiation detector comprising:

an envelope including a portion which is substantially transparent to ultraviolet radiation; an anode and cathode closely spaced with respect to one another and spaced at a relatively large distance with respect to the interior surfaces of the envelope; and

an ionizable gas filling having a low breakdown voltage, a high average ionization efficiency in a region proximate the anode and cathode, and a low aver- I age ionization efficiency in other regions within the envelope, the ionizable gas filling comprising a gas mixture of neon and one or more of the following: argon, krypton, and xenon; wherein the gas mixture substantially determines the breakdown voltage and ionization efficiency characteristics of the ionizable gas filling, and

an amount of hydrogen sufficient to rapidly deexcite argon, krypton, or xenon metastable atoms. 2. The ultraviolet radiation detector of claim 1 wherein the hydrogen partial pressure exceeds about 0.01 torr.

3. The ultraviolet radiation detector of claim 2 wherein the ionizable gas filling comprises less than about 15 mole per cent hydrogen, between about 8 mole per cent and about 60 mole per cent argon, and balance essentially neon; and wherein the percentage of argon is at least twice the percentage of hydrogen.

4. The ultraviolet radiation detector of claim 3 wherein the ionizable gas filling comprises less than about 12 mole per cent hydrogen, between about 30 mole per cent and about 50 mole per cent argon, and

balance essentially neon.

* =l =l l 

1. An ultraviolet radiation detecTor characterized by its low spurious count rate and prolonged lifetime at elevated temperatures, the ultraviolet radiation detector comprising: an envelope including a portion which is substantially transparent to ultraviolet radiation; an anode and cathode closely spaced with respect to one another and spaced at a relatively large distance with respect to the interior surfaces of the envelope; and an ionizable gas filling having a low breakdown voltage, a high average ionization efficiency in a region proximate the anode and cathode, and a low average ionization efficiency in other regions within the envelope, the ionizable gas filling comprising a gas mixture of neon and one or more of the following: argon, krypton, and xenon; wherein the gas mixture substantially determines the breakdown voltage and ionization efficiency characteristics of the ionizable gas filling, and an amount of hydrogen sufficient to rapidly de-excite argon, krypton, or xenon metastable atoms.
 2. The ultraviolet radiation detector of claim 1 wherein the hydrogen partial pressure exceeds about 0.01 torr.
 3. The ultraviolet radiation detector of claim 2 wherein the ionizable gas filling comprises less than about 15 mole per cent hydrogen, between about 8 mole per cent and about 60 mole per cent argon, and balance essentially neon; and wherein the percentage of argon is at least twice the percentage of hydrogen.
 4. The ultraviolet radiation detector of claim 3 wherein the ionizable gas filling comprises less than about 12 mole per cent hydrogen, between about 30 mole per cent and about 50 mole per cent argon, and balance essentially neon. 